1. Field of the Invention
The present invention relates generally to extrusion screws. More specifically, the present invention relates to injection molding screws.
2. Description of the Related Art
Extrusion screws and extrusion devices or systems are known in the art.
A known Trexel, Inc.-designed injection molding screw used in the MuCell® process for polymer materials is not an optimal design for processing metal or ceramic feedstock materials and the like. This screw has four distinct sections.
The first section, starting from the drive or hopper end, includes a feed section, a transition section, and a metering section.
The second section, also referred to as a restriction element, has an increased minor diameter, and is located between the conventional screw flights and the first mixing section of the standard screw for the microcellular process. The second section restricts the back flow of molten material upstream of the gas port location. This feature is for melt pressure retention when the high-pressure gas is added to the molten material in the barrel.
The third section is a reverse channel flow mixing section, based on a Union Carbide or Maddock mixing design, which is commercially available from industry injection molding machine screw suppliers. This section coincides with the gas port orifice for the introduction of super critical fluid (SCF) or blowing agent, such as nitrogen, carbon dioxide, and other atmospheric gases, into the molten material. The flow of material through this section exposes a large area of molten material to accept the SCF or blowing agent to promote uniform distribution and dispersion into the molten material.
The fourth section is a close pitch, dual flight mixing section. The flights have unequal height and are segmented to promote the cross flow of molten material between the flights of this mixing section.
This known standard screw for the microcellular process for polymer materials has design limitations when processing metal or ceramic feedstock materials and the like.